Field of the Invention
The present invention relates to a position detection apparatus which detects a position of an object.
Description of the Related Art
Previously, an encoder is used as a position detection apparatus provided in a machine tool, an FA equipment, or the like. In an incremental encoder, a reference point is detected by using a reference position mark provided at a specific position on a scale, and a position within a stroke is detected based on a moving amount relative to the reference point.
Japanese Patent Laid-open No. H10-300518 discloses a method of detecting a reference point on a scale. In the method disclosed in Japanese Patent Laid-open No. H10-300518, a solitary wave which is generated at a reference position on the scale is used, and a reference position pulse is generated based on a result obtained by comparing the solitary wave with a threshold value. Furthermore, a level of the threshold value changes with the increase or decrease of a direct-current component of a light amount.
However, in the method disclosed in Japanese Patent Laid-open No. H10-300518, a pulse position or a pulse width of a reference signal which indicates the reference position varies when a ratio of the direct-current component of the light amount and a modulation amplitude of the solitary wave is changed. As the direct-current component of the light amount, there are an external light component, an internal reflection of a sensor, a scatter component, and the like, in addition to a reflection component from the scale. Accordingly, since the ratio of the direct-current component and the modulation amplitude of the solitary wave varies depending on a change of an intensity distribution with the change of an environmental temperature or a state of a brightness, it is difficult to detect the reference position with high accuracy.